This invention relates to the art of pressure sensors and, more particularly, to a catheter tip pressure transducer of sufficiently small size that it may be employed for measuring fluid pressure, such as blood pressure, within the human body.
Although the invention will be described in detail herein with respect to its employment in measuring blood pressure, it is to be appreciated that the pressure transducer may be employed within a hollow needle for use in measuring pressure at remote locations requiring transducers of exceedingly small size, i.e., of a size sufficiently small that it can be inserted into a blood vessel or the like.
Catheters have been used in the art for monitoring variations in blood pressure within a blood vessel, such as within the cardiovascular system. Such catheters include those employing catheter tip transducers insertable into a blood vessel with the transducer providing direct pressure monitoring by transducing blood pressure at the region of interest. Such a catheter tip transducer may employ semiconductor material constructed and arranged with resistors and the like for use in developing an electrical signal representative of the monitored pressure and transmitting the signal by electrical conductors through the length of the catheter to meters or the like located externally of the body being tested.
An example of such a catheter tip pressure transducer is found in the Mizuno et al., U.S. Pat. No. 4,274,423. The transducer disclosed there includes a pressure sensor disposed within the end portion of a catheter. The pressure sensor takes the form of a pressure sensitive diaphragm constructed from a block of semiconductor material, such as silicon. The diaphragm is located adjacent a side port in a housing connected to the end of the catheter with the side port providing access to the pressure medium. The diaphragm is deflected in dependence upon the pressure and the deflection is sensed by one or more strain gauges located in the diaphragm. The strain gauges are connected by suitable conductors to a meter located outside of the catheter.
A major concern with such catheter tip pressure transducers is to provide a transducer which is sufficiently small so as to be employed in a catheter adapted to be inserted into a blood vessel of a patient, while also being sufficiently responsive to pressure variations to provide meaningful electrical output signals. For example, the preferred outside diameter of such a catheter may be on the order of 2 or 3 millimeters. Consequently, the cross sectional dimensions of a pressure sensor of the type employing a membrane or diaphragm as discussed above must be optimally small. The semiconductor block containing the diaphragm or membrane of the type employed in Mizuno may well have a width on the order of 1.2 millimeters, which limits the size of the catheter and, hence, its application for use in measuring blood pressure within a blood vessel.
It is, therefore, a primary object to provide a catheter tip pressure transducer exhibiting cross sectional dimensions which are reduced over that of the prior art without impairing its operation.
To achieve the foregoing objective in accordance with the present invention, the catheter tip pressure transducer includes a tubular housing having a pressure inlet aperture proximate to one end thereof. An elongated block of semiconductor material is mounted within the housing and has first and second oppositely facing surfaces, one of which faces the inlet aperture. The block has a rectangular recess located in the first surface. The floor of the recess together with the second surface define a relatively thin rectangular flat portion having a length l and a width b and a thickness d. A pair of longitudinally extending grooves are formed in the flat portion of one of the surfaces thereof at a location corresponding with the longitudinal edges of the rectangular flat portion. This leaves an elongated rectangular membrane beam of a width s and a length l and a thickness d supported at its free ends by the semiconductor block and relatively unsupported along its longitudinal edges. The amount of strain in the beam in response to applied pressure is mainly determined in the length and the thickness of the beam and relatively not by the width allowing the width to be small without impairing sensitivity. At least one strain gauge is mounted on the beam, which serves as a deformable membrane for providing electrical signals by way of suitable electrical connecting means through the catheter to a meter or the like located externally thereof.
In accordance with another aspect of the present invention, the longitudinally extending grooves bordering the membrane beam extend through the rectangular portion defining two elongated openings or slots along the longitudinal side edges of the beam.
In accordance with a still further aspect of the present invention, the openings along the longitudinal side edges of the beam are closed with a thin layer of polymer permitting a pressure differential to exist across the oppositely facing surfaces of the beam.